1. Field
The present disclosure relates generally to wireless devices, and, more specifically, to systems and methods for determining and calibrating DC offset in a wireless device with the antenna connected.
2. Related Art
A wireless network generally includes two or more wireless devices that communicate with each other over a wireless medium. One example of a wireless network is a wireless local area network (WLAN) designed to operate according to IEEE 802.11 standards.
DC offset is usually undesirable because it causes saturation or change in the operating point of an amplifier in the wireless device. When a receiver (or wireless chipset) is integrated in a final product and connected to an antenna, it is exposed to all kind of interferences from other transmitters, microwave ovens, other appliances, and the like. DC Offset is particularly sensitive to input impedance. Although a manufacturer of a wireless chipset could calibrate the chipset itself, the final product would still suffer from DC offset because the impedance of the antenna is different for each product. In addition, DC offset can also be affected whenever there is saturation caused by interferences or when there is interference at very low frequency offsets from LO frequencies.
Examples of factors that affect DC offset measurements include the non-linear effect of high power jammers, carrier leakage or subcarrier close to receiver local oscillator (LO) frequency, leakage power that is smaller than total interference power, leakage phase that is zero-mean random process, transmitter and receiver are asynchronous, and the delay in propagation is random. Depending on the LO leakage power level, all receivers suffer from DC offset caused by self-mixing of the LO with its leakage. Therefore, DC offset in the receive paths varies across the designs as the antenna impedance changes. It is impossible and impractical to predict the impedance in the final products; thus, it is impossible to predict the DC offset in the final products. This uncertainty causes a problem in determining DC offset cancellation.
While FFT based spectrum analysis may allow for detection and elimination of some interferences, it requires higher resolution and very complicated algorithms to analyze the spectrum. Additionally, manufacturer calibration is expensive, and it is even more expensive to calibrate over variations in temperature.